It has been recognized that by injecting and releasing steam into and out of composite products during consolidation thereof using heat and pressure, several improvements can be achieved, a main one of which is the increase of heat transfer rates which significantly speeds up curing of thermosetting adhesives. Several methods and types of apparatus have been proposed to achieve this objective.
Corbin U.S. Pat. No. 3,295,167 discloses a steaming apparatus for consolidation of composite products, composed of a source of superheated steam which is fed into a platen having a chamber and a plurality of openings communicating the chamber with the material undergoing consolidation. By passing through and out of open pressed products, steam speeds up heat transfer and curing of thermosetting resins.
Futo U.S. Pat. No. 3,619,450 proposes a gas-tight envelope made of Teflon sheet, reinforced in suitable manner, surrounding press platens with pressed products therebetween, for the purpose of controlling the ambient atmosphere in and around the pressed products.
Shen U.S. Pat. No. 3,891,738 shows a press platen which, in addition to providing conventional internal heating by passing heating fluids through a closed labyrinth of interconnected passageways, has an additional chamber and aperture openings on the surface adjacent to the product undergoing pressing for injecting steam into the product. Steam passes from a chamber of one press platen through openings into the pressed product and from there into the opposite press platen, thus speeding up curing of thermosetting resins.
Nyberg U.S. Pat. No. 4,162,877 discloses one platen almost identical to that of Shen, instead of two platens with chamber and aperture openings on the surface which comes into contact with the product. Steam is injected from the press platen through the openings into the product and released back through the same openings into the platen after curing the thermosetting resin in the product.
The Makinen U.S. Pat. No. 3,686,383 discloses a system involving two stages of pressing. In the first stage, steam is withdrawn from an open press, while in the second stage the product is pressed in a closed press for a time of up to thirty-eight minutes.
In all the known prior systems which attempt to utilize the positive effects of steam injection into products undergoing consolidation, the prior workers have invariably provided special presses and have failed to produce a simple and dependable means of converting an existing press to steam injection pressing. Thus, methods of the above-described prior art require complex press platens which are, in fact, double platens in the sense that they possess double systems, i.e. a first conventional passageway system for heating fluids by conduction heating; and an open passageway system for steam injection heating. In some cases a new platen has been added to a conventiona platen so that each press platen is composed of two separate platens, one conventional for conduction heating and the other especially designed for steam injection heating.
Thus, a conversion of an existing conventional conduction press to a press for simultaneous conduction and convection heat transfer requires the replacement of existing press platens by such double platens which are more complex and more expensive. Because such platens are heavy, replacement is time consuming and is associated with substantial losses of production time, to say nothing of capital costs. When it is realized that some presses are as large as 8 feet by 60 feet, e.g. for making mobile home walls, it can be readily understood that just the capital costs involved in the provision of new presses can be very substantial.
Moreover, press platens for such dual heat transfer as proposed in the prior art, i.e. both conduction and convection, are not dependable. Passageways in platens for steam injection inevitably become filled with deposits from binders and wood extractives which block the passageways, particularly after considerable periods of continuous operation. It is very difficult, if not virtually impossible, to clean the passageways so that they function properly. Therefore, providing such very expensive dual function plattens is not economically feasible.